A player in my dnd group wants to add his wife to the game, is this a good idea? We already have 5 players and play online through discord voice chat. 

It *highly* depends. Couples can game together just fine; there's slightly more risk if the DM has a significant other in the party, but it's something that can be overcome.

If the wife is an avid player of the game, then you're likely fine. If they just want to do things together, then it *could* go sideways if the wife decides she doesn't like the group/etc - but that's likely to happen anyways if she's rejected. If she's a new player, then you'll have a new player on your hands - and if she's just there to do something with her husband, then it's possible she won't put much effort into learning the game/etc.

Another concern is that you already have five players; six might be pushing your comfort bounds. Every time a player is added the entire game slows down significantly - keep that in mind. But that's really just a 'if five is the number of players you wanted, then adding another one disturbs that desire' answer - it's nothing specific with having a significant other in the group.

Basically this all boils down to "does she have a significantly different motivation for wanting to be in the group than the rest of the players? If so, then you *might* have a problem depending on how she handles things".

Well she is completely new to the game, so I'm hoping she can understand that characters are not players and vice versa.

I can only mirror LangyMD's views really. It depends entierly on the people. But in the planeshift campaign I'm running, I have a couple playing, and they are fine. To be fair though, I invited them both together as the guy of that relationship already runs a 5e campaign with us and a circle of friends, so they are both heavily into D&D.

In the two games I DM. 

Game one my wife is a player and there is a couple.
Game two, there is a couple in that game.

Players are players they fit or they don't their relationship to each other shouldn't make a difference. Likewise it will be up to you if you can cope with the player count.

However if you don't add the other half will the other player feel less inclined to play, if the other half goins the attendance might be higher.

I find couples work well together as their characters often gell and they can work with each other on rules and tactics outside of the game. 

This isn't like normal board games where couple either work as one unit or go hard against each other than dynamic shouldn't affect an cooperative RPG

While the above concerns are possible, the only issue I have ever seen is when the *in game character* of one spouse has an *in game* relationship with another *in game* character, that is not the spouse's character.  If everyone is comfortable with this concept, you get Vax/Kiki and Vex/Percy from Critical Role.  If the new spouse is not aware of an existing *in game* relationship, and/or is uncomfortable with the concept, that is going to introduce some toxicity to the game, and understandably so.  Happens to professional actors/actresses all the time, because even fake relationships can seem real.

If your current player is not in this situation, or your game does not focus on that style of RP, you should be fine.  If he is/you do, I recommend requiring him to explain this to his wife first, so she can then decide if that is ok for her.  Worst case scenario, she's super uncomfortable, and you lose a player (who should have been up front with his wife in the first place).  Neutral scenario, she says no, and you keep the current party.  Best case, she's cool with it, and you guys gain another player.

As far as going from 5 --> 6 players, anything 4 --> 7 players doesn't really impact dm management (unless they are disruptive, which is separate issue).  For some reason, from what I've run/played/observed, once you hit 8-10, it gets rough.  Anything more than 10 is just silly, imo.

I should add, for the 4-->7 players comment, while adding players mentally seems huge, it actually integrates well.  For some reason, 8 is the magic number where fewer DMs are comfortable doing it, and the work seems to ramp up.

And, like I said, over 10 is silly, or a special event like an in-store Battle Royal.

My husband and I have gamed together successfully for 20 years and we have met many gaming couples through those years. Allowing couples into your game can be a mixed bag. Awkward situations can present themselves but as long as you (as the DM) keep control of the game all issues tend to work themselves out. Honestly, I would be more concerned about the size of group you can handle. 

I have DM'd with my wife as a player and other then her pouting when I wouldn't grant her bonus stuff which she really didn't mind, and I have played with others that had couples in the group only issues was when the couples didn't get along and one of them trying to pull the group into the difference. I as a player had to say I would not be taking sides at the gaming table. That would usually end it as other players would support that and stay out of it as well.

I've had many couples in my games. They are usually fine. You'll see some who have their characters be close to each other and others who play separate lives. My plays in my game every so often and when a few bad dice rolls lead to her character dying she was ok with it. The rest of the party went way out of their way to get her a rez but I think they would have done the same regardless of who died.